Humidity sensors are formed by two armatures, each provided with a plurality of arms. The two armatures are designed to be interleaved with each other and have a dielectric between each other. These armatures are supplied with voltage so that their interleaving causes the creation of a capacitance.
This humidity sensor functions such that, when the degree of moisture in the air increases, molecules of water infiltrate between the two armatures into the dielectric. This infiltration of water causes a modification in the capacitance which makes it possible to quantify the degree of moisture during a measurement by applying a voltage. The humidity sensor can be a structure connected to an integrated circuit or be directly included in the integrated circuit.
Several methods are known for determining a humidity value from this capacitance.
A first method consists of connecting, at the output of the humidity sensor, a charge amplifier in order to obtain a voltage signal which is representative of the degree of moisture. This signal is sent to a signal conditioner which prepares the signal to be sent to an analogue-digital converter so as to provide a numerical value N. This numerical value N is then sent to a circuit which linearises it because the curve of the capacitance as a function of the degree of moisture is not linear. A numerical value N which is a function of the capacitance C of the sensor is then obtained.
A second method consists of connecting, at the output of the humidity sensor, a circuit in order to obtain a frequency signal which is representative of the degree of moisture. In fact, this variable capacitance is inserted in a resonator so as to produce an RC oscillator. Consequently, the frequency output from said resonator is dependent upon the value of the capacitance and therefore upon the degree of moisture. This frequency is sent to a signal conditioner which prepares the signal to be sent to an analogue-numerical converter so as to provide a numerical number N. This signal conditioner can be a numerical frequency demodulator or an assembly formed by a circuit which makes it possible to convert this frequency signal into voltage then to convert it into a numerical value N which will be processed in order to obtain the value of the physical quantity.
A disadvantage of these two methods is that they require a circuit which comprises a large number of components, i.e. a circuit, the manufacturing cost of which is high but also which requires a larger silicon surface during integration in an integrated circuit. For this reason, the number of measuring circuits per circuit board is less.